DE10356703A1 - Method of burning fossil fuels in steam generator using oxy-fuel process, by using waste gas to preheat oxygen used in combustion - Google Patents

Abstract

The oxygen used in the combustion of the fossil fuels is preheated by being used to cool the waste gas. The quantity of recirculated waste gas is increased. A gas-tight heat exchanger is used for preheating the oxygen, e.g. a plate- or pipe-based heat exchanger.

Description

The The invention relates to a method of burning fossil fuels in a steam generator after the oxyfuel process.

at the combustion of fossil fuels for the production of electrical energy Energy is the flue gas loaded with many pollutants. That's why the power plant process u. a. Downstream of flue gas purification systems, to reduce the pollutant content in the flue gas. This is one of the secondary measures which makes the whole energy conversion process more environmentally friendly shape.

However, primary can already taken action be to the formation of the appropriate pollutants in the combustion process largely to minimize.

In recent years, the civilization-related emissions of CO ₂ as a presumed major cause of the climate-changing greenhouse effect has increasingly come under public scrutiny.

In the meantime, the Kyoto Protocol has also made manufacturers and operators of fossil-fueled power generation plants responsible for providing concepts for reducing specific CO ₂ emissions. For example, most of the CO ₂ produced in Germany comes from fossil-fueled power plants and industrial plants.

However, a further increase in the conversion efficiencies, ie a reduction in fuel input while maintaining a high output, can at best cause a stagnation of the amount of CO ₂ emitted annually in view of the increasing energy demand. For a significant reduction under adherence to fossil fuels, therefore, only a separation and climate-effective landfilling of the generated CO ₂ comes into question.

Assuming that the currently investigated methods for transport and storage of CO _{2 are} suitable for large-scale use, the so-called oxyfuel process represents a promising power plant concept, which instead of the emission of flue gases, a CO ₂ stream of high purity can provide.

at a thermal power plant After the oxyfuel process, the combustion does not take place with the help of of combustion air, but by the previously separated from the air Oxygen stream. Since only with oxygen is burned, also occurs in total less flue gas.

The Heat this Flue gas on the low temperature side, d. H. after passing the steam generator pressure part heating surfaces, can in principle be exploited by the feed water preheated with the flue gas (Flue gas heated feedwater pre-heater). This circuit has however, certain thermodynamic disadvantages.

by virtue of the use of heat of the flue gas for feedwater preheating must be less tap from the turbine for the preheating of the feed water are spent and this saved tapping steam stands for one Work performance in the turbine and thus a higher energy gain available. This But thermodynamically is not optimal insofar as it is in the Tapping steam is at low pressure steam.

One Another problem with the oxyfuel process is the high adiabatic combustion temperatures.

So is out of the DE 197 03 197 discloses a method and apparatus for burning wood and / or biomass in a circulating fluidized bed reactor with pure oxygen. Again, the problem of high temperatures in the furnace is mentioned, which despite frequent refractory materials for longer periods of use frequent maintenance and repair work. Furthermore, there are problems with contamination and slagging of heating surfaces.

It It is also known that this circumstance can be counteracted. This is cooled flue gas fed back to the circulating fluidized bed, so as to reduce to reach the process temperature.

Of the Invention is based on the object, the thermodynamic disadvantage the insufficient heat utilization the flue gas during preheating of the feedwater.

The The object is achieved in that the oxygen used to burn fossil fuels by cooling the flue gas preheated becomes.

Advantageously, the amount of recirculating flue gas is increased to the additional heat input into the steam generator the temperatures continue to dominate. Gas-tight heat exchangers (eg plate or tube heat exchangers) should be used as heat exchangers for oxygen preheating.

You cool the flue gas instead of a feedwater-heated preheater thereby From that one preheats the oxygen flow, one achieves by the thus higher heat introduced into the boiler a higher one heat flow in the steam generator and can thus generate an increased live steam flow, which compared to the above-mentioned extraction steam flows higher steam parameters and thus a higher one enthalpy in the turbine. Thus one achieves with such a preheater in principle a higher one thermodynamic efficiency of the process.

As shown above, arise in the oxyfuel process without other Interventions very high temperatures and you have a corresponding amount of flue gas recirculate to bring the temperature back to tolerable levels (For example, with regard to pollution and slagging of steam generator heating) get. An additional preheating Of the oxygen, as proposed here according to the invention, the combustion temperature is still increased more and appears first absurd.

As detailed studies show, the O ₂ preheating leads to an improvement of the power plant efficiency of about 0.5%, even if this initially required temperature-reducing measures (increased recirculation).

However, it is possible to compensate for the increase in the flue gas temperature associated with an additional O ₂ preheating by a relatively small increase in the abovementioned recirculation of the flue gas, so that overall no special disadvantage arises and the advantages of the O ₂ preheating predominate (a Increasing the recirculation means only a slight increase of the own consumption, but is not a significant disadvantage in itself from the thermodynamic cycle process).

The O ₂ preheating is realized in such a way that the oxygen coming from the air separation plant is preheated by the existing flue gas.

The circuit can also be multi-stage to first heat the O ₂ in a low flue gas temperature stage and then in a higher flue gas temperature stage, thus preventing excessive temperature differences on the cold side of the O ₂ pre-heater, resulting in exergy loss would mean. The O ₂ preheater can be designed as a regenerative preheater.

Since this is probably due to the need for the lowest possible O ₂ -Leckagen (from the O ₂ side in the flue gas), either the leaks are to be kept low or to make the pressure conditions so that the leakage in the direction of O ₂ runs, ie Leakage of CO ₂ from the flue gas side to the O ₂ side or that a corresponding smoke gas-tight O ₂ preheater (eg a tube preheater or plate preheater) is used.

Another (additional) possibility is to pre-heat the oxygen in a first stage or in one of the first stages of the oxygen preheating by cooling the air separation plant compressors or the CO ₂ compression or with tapping steam from the turbine.

At an embodiment the invention will be described in more detail below explained become. The figure shows a schematic representation of a simplified Heat diagram a steam power plant after the oxyfuel process.

For a lignite-based steam power plant with an output of about 900 MW are used for combustion in the steam generator 1 instead of air, the oxygen separated from the air 7 provided in a purity of 99.6%. This happens z. B. using a cryogenic air separation plant. This cryogenic air separation plant splits the airflow drawn from the environment into a nitrogen stream and a high purity oxygen stream.

That in the 1 shown simplified heat circuit diagram of a steam power plant after the oxyfuel process has a steam generator 1 on, the fresh steam side with a turbine 10 is connected, in which the live steam relaxes and gives the work done to a generator not shown in the drawing. The live steam temperature is about 600 ° C at 290 bar. An illustration of a reheat was omitted for reasons of simplification.

In the pressure variable taps each feed a Hochdruckvorwärmsäule 5 and a low pressure preheating column 4 ,

The leaving the turbine exhaust steam is in the condenser 2 condenses and the condensate passes through the flue gas heated condensate preheater 3 , the low pressure preheater column 4 and the high pressure preheater column 5 back to the steam generator 1 for fresh steam generation. The condenser operates at 40 mbar and gives heat to one Cooling tower (not shown in the figure).

The condensate preheater 3 lowers the temperature of the flue gas entering a flue gas desulphurisation plant 8th to about 80 ° C from.

The steam generator 1 becomes lignite for burning 6 and as the oxidant oxygen 7 supplied from the air separation plant. The resulting flue gas 8th is recirculated to a part and thus the steam generator 1 via a recirculation fan 11 abandoned again, on the other hand an oxygen preheater 9 fed.

The recirculation rate of the flue gas 8th is about 73%, whereby the recirculation itself can be hot or cold. The flue gas leaves the steam generator 1 with a temperature of about 340 ° C.

Farther it is also possible the benefits of cold and hot Recirculation of flue gas to connect together and use a recirculation preheater (Flue gas / flue gas).

The oxygen preheater 9 transmits heat at least a partial stream of the flue gas 8th on the oxygen 7 , As an oxygen preheater 9 a gas-tight plate or gas-tight tube preheater can be used.

With a minimum rate of 40 K in the oxygen preheater 9 results in a temperature of the oxygen 7 after oxygen preheater 9 of about 300 ° C. The thus heated oxygen now passes to the combustion in the steam generator 1 , To prevent too high adiabatic combustion temperatures due to the additional heat input with the preheated oxygen 7 there is an increase in the recirculation rate of the flue gas 8th on the order of 0.5%.

After passing through the flue gas 8th through the oxygen preheater 9 this still has a temperature of about 195 ° C. This is enough to remove the condensate in the condensate preheater 3 to warm up to approx. 45 ° C. The oxygen preheating can be done in several stages. In addition to the preheating of the oxygen 7 in the oxygen preheater 9 is it possible that the oxygen 7 in a first stage or one of the first stages of the oxygen preheating is preheated by cooling of compressors of an air separation plant or a plant for CO ₂ compression and / or with bleed steam from a turbine.

Of the Overall efficiency of the power generation plant after the oxyfuel process increases with the inventive solution about 0.5%.

One decisive reason for this is the reduction in the proportion of the fuel heat output to the primary boiler heat output by the oxygen preheating (Primary Boiler output = Fuel heat output + Heat content the burner gas).

These Partial shift is also the reason that for the variant with oxygen preheating the primary Boiler output while higher, the fuel heat output on the other hand is lower than in a circuit without oxygen preheating. exergetic expressed can the available standing flue gas energy most profitable at the high temperature level used for combustion, whereas when used for feedwater pre-heating only exergetisch low-quality bleed steam is saved.

The changed concept of the water-steam cycle when using the oxygen preheater has favorable effects on the net efficiency of the plant. An essential feature of the circuit with oxygen preheater is the about 20 MW _th lower heat loss through the cooling tower.

This will - despite the higher one Live steam production - explainable by the significantly reduced Abdampfmassenstrom from the low-pressure turbine to the condenser. The difference between live steam and exhaust steam results from the increased demand for bleed steam for feedwater pre-heating, since the flue gas heated condensate preheater by adding the Sauerstoffvorwärmers a lower warm-up period achieved.

Out It can be generally concluded from the explanations that an oxygen preheating during Oxyfuel process leads to a significant increase in efficiency. One important point is first, that is an increase the proportion of fuel gases in the primary boiler heat output on favorable affects the fuel consumption and thus the plant efficiency, d. H. oxygen preheating and as possible High temperatures of the recirculated flue gases are desirable.

Of more, it is important to use steam-fed feed water heaters, preferably a lot of tapping steam only in the last stages of the low-pressure turbine deduct, because this then on the one hand in the high pressure stages a maximum of mechanical Power can generate, on the other hand, but also the condensation heat of the Steam is not delivered in the condenser, but sensibly used to feed sewasservorwärmung is, d. H. preferably low condensate temperature when entering the preheating column.

Both goals are through use an oxygen preheater in an oxyfuel process with extensive waste heat recovery (strong flue gas cooling) even better met and the efficiency can be increased by up to 0.5 percentage points.

1

steam generator

2

capacitor

3

condensate

4

Niederdruckvorwärmersäule

5

Hochdruckvorwärmersäule

6

Brown coal

7

oxygen

8th

flue gas

9

Sauerstoffvorwärmer

10

turbine

11

recirculation

Claims (5)

Process for burning fossil fuels in a steam generator after the oxyfuel process, characterized in that the oxygen used to burn the fossil fuels by cooling the flue gas preheated becomes. Method according to claim 1, characterized in that that the amount of recirculated flue gas is increased. Method according to claims 1 and 2, characterized by that as a heat exchanger for the oxygen preheating gas-tight Heat exchangers used become. Method according to claim 3, characterized in that that as a gas-tight heat exchanger Plate or tube heat exchanger be used. Method according to one or more of claims 1 to 4, characterized in that the oxygen is preheated in a first stage or one of the first stages of oxygen preheating by cooling compressors of an air separation plant or a CO ₂ compression and / or with bleed steam from a turbine ,